Methods and apparatus for melting glass in a furnace, hereinafter referred to as a melter, and conveying the molten glass out of the melter through a bushing for delivery to subsequent processing apparatuses are well-known. In general, pre-mixed glass batch materials are fed into a melter where the materials are melted into glass by heat sources such as gas flame, electric arc, or electrical resistance. The molten glass is usually passed from a melter through a conditioning chamber to a forehearth, where the glass is maintained in a molten state.
The molten glass is conveyed from the container such as the forehearth by bushings that are mounted to the bottom wall of the forehearth. A stream of glass issues from the bushing and falls into or onto subsequent processing apparatuses such as a glass fiber centrifugal spinner, or into a cullet chute if the molten glass is not to be processed further.
Typical forehearths in the glass fiber industry are lined with refractory material and configured to maintain a depth of about six to eight inches of molten glass above the refractory floor. The bottom wall is about five inches thick, and the refractory floor contains bushing wells onto which are mounted the bushings that convey the molten glass from the forehearth to spinners or to cullet chutes.
A forehearth performs a number of functions, such as:
conveying molten glass from the melting furnace to a subsequent processing location, PA1 providing a smaller, hence less hazardous hydraulic head over the bushing than does the melting furnace, PA1 uniforming the glass temperature by mixing the glass, destroying temperature stratification, and PA1 allowing bubbles to rise out of the molten glass. PA1 Refractory debris called stones travel along the floor of the forehearth into the well and through the bushing to the spinner, affecting the processing and quality of the glass fibers. If large enough, the stones may block the bushing and affect throughput. PA1 Power and power equipment are costly. PA1 Power outages will decrease or stop throughput. PA1 Power equipment can present an electrical hazard in the wet environment of glass processing operations. PA1 The bottom glass in the forehearth is not as hot nor as clean as the glass at the top surface of the glass in the forehearth.
The level of molten glass in a forehearth is determined by the mass flow rate from the melter, and by the mass flow rate from the bushings, called throughput. The flow rate from the melter is controlled by controlling the batch feed rate to the melter, or if so equipped, by a needle throttling device on the melter. The forehearth level is usually sensed by a pressure differential device that utilizes an air bubble generator to propel air through a tube below the surface of the melt, causing bubbles to rise up through the molten glass. The pressure required to force the bubbles out of the tube is proportional to glass depth, and the pressure signal is used to control batch feed or needle valve setting.
Bushings used in the glass industry are usually fabricated from heat resistant precious metal alloys and configured with lugs or ears to which are connected electrical power clamps. The bushing is encased in a protective refractory insulation casing with the ears protruding through the refractory casing. Varying the power to a bushing will vary the temperature of the bushing and therefore the temperature of the glass at or in the bushing. Raising the power level will decrease the glass viscosity, increasing the throughput. Lowering the power level will have the opposite effect. Accordingly, throughput can be throttled by bushing power setting. Bushing throughput can be reduced to zero by lowering the power setting sufficiently.
Electrically powered bottom-feed bushings mounted to the floor of the container have certain disadvantages:
This invention overcomes these disadvantages of bottom feed, electrically powered bushings and demonstrates additional utility not available in these bushings.